1. Field of the Invention
The present invention relates to a mask layout and exposing device in the process of semiconductor production, and more particularly to a symmetrical layout using pattern of a single mask and the method of the same.
2. Description of the Prior Art
During the last decade, due to the prospering wireless communication industry, the band of electromagnetic wave used has reached that of microwave, 1 GHz˜300 GHz. And under the trend of minimization of products, the need of microwave elements, such as filter, surface acoustic wave device, and spiral inductor on some active elements, has increased. With the aid of fast-developing technique of semiconductor production, the trend drives on the mass production and low price of microwave elements, matching the elements with the commercial demand at a high speed. However, as the frequency range of microwave elements increases, concerning product integration and cost of production equipment, there are some lithography problems encountered in the process of microwave element production. The most difficult one is diffraction effects produced in exposing process.
Because of diffraction effects in the lithography process, in addition to the normally incident rays of light, some light propagates at divergent angles. As device geometries shrink, the phenomenon will result in sufficient resolution. Therefore when tiny pattern is exposed, there will be incomplete exposure on photoresist formed on the chip, and due to the spread of illuminant energy, there will be a chemical change on the photoresist, which, originally, does not need exposure. It will result in organic residue or incomplete shape of overhang or T-Top after development process, and thus affects following lift-off process. The organic residue may cause poor adhesion of thin film, such that the metal film peels off easily. The lift-off may be incomplete and thus causes residual metal film. In these situations, the elements will have short circuits or incomplete contacts, and thus become failure products.
Take a surface acoustic wave device (SAW) that has the simplest process of production for example. Surface acoustic wave is an elastic wave that spread along the surface of a solid body, the elliptical locus of which is composed by longitudinal wave and shear wave. Its largest amplitude is on the surface of a solid body and declines exponentially as the wave goes deeper, so 90% of the mechanical energy it transmits centers on the depth of about one wavelength. The basic functioning principle of SAW is to transduce input electric signal into acoustic signal by interdigital transducer (IDT) through reverse voltage effects and the acoustic signal is transmitted along the surface of piezoelectric substrate and then transduced into output electric signal by IDT through positive piezoelectricity effects. Thus SAW is a signal processing device on piezoelectric substrate using the principle of transducing acoustic and electric energy.
The electrical performance of SAW basically corresponds with geometrical pattern of IDT, the center frequency of which depending on the width of interval of periodic IDT, the phase of which corresponding with the position of IDT, and the amplitude of which corresponding with the length of overlaying of IDT.
The focus of discussion is then concentrated on the distance between adjacent IDT. According to the above mentioned, the center frequency of SAW depends on the interval of IDT. Generally speaking, it can be decided by the following equation,V=f0λ, in which                V is the SAW acoustic velocity of piezoelectric substrate;        f0 is the center frequency of SAW device;        λ is the wavelength of SAW device.        
For a SAW filter operating at 2.5 GHz, with piezoelectric substrate Lithium Tantalate having an acoustic velocity of about 4000 m/s, the wavelength (λ) of said SAW filter is about 1.6 μm based on the above equation. The width of interval of adjacent IDT on SAW filter (d) is generally designed to be one-fourth of the wavelength (λ/4), therefore the interval of adjacent IDT can be further figured out to be 0.4 μm. If the center frequency of SAW filter is designed to be higher, for example, 5 GHz, then under the premise that the conditions above do not change, the interval of adjacent IDT of SAW filter will be down to 0.2 μm.
In semiconductor industry, it is very common to fabricate 0.4 μm (even lower) linewidth device by using advanced stepper, but it is very difficult to justify to invest such expensive stepper for economically manufacturing SAW devices. Hence, so-called cost-effective I-Line stepper with optimum resolution 0.4˜0.7 μm became the main exposer in SAW industry. However, when to produce a SAW filter having the center frequency of 2.5 GHz by I-Line stepper, there will be diffraction effects in the exposure, so decreases the yield of products.
In order to overcome the exposing restrictions imposed by diffraction, a “Method of Making Surface Wave Devices” is disclosed in U.S. Pat. No. 5,972,568, the feature of which is to divide IDT on surface acoustic wave into two subsets and provide the two subsets on the same mask. The fabrication methods can be illustrated by FIGS. 1a to 1c. 
Typically, the lithography process is to divide a wafer into a plurality of shots, then the exposure is proceeded along X axis or Y axis by exposer, and one exposure is made through projection each time the exposer moves for a distance of one shot. In the producing method FIG. 1a, the first part of IDT pattern on reticle will be exposed one shot after another by blinding the second part. After the exposure of all shots is completed, the exposer will return to the starting position (first shot) and then the second part of IDT pattern is exposed one shot after another by blinding the first part. Finally, the exposure of the whole IDT pattern of surface acoustic wave element is completed, and the following procedure is then proceeded. Since the first and second parts symmetrically divide IDT pattern into two parts, the interval of adjacent IDT on the first and second parts is increased from 0.4 μm to 0.8 μm. As the interval of adjacent IDT increases, the diffraction effects can be reduced effectively, but time for exposure also doubles. Besides, it is emphasized in the patent that the method claims precise alignment and does not have the problem of overlaying. In fact, owing to that the exposures of the first and second parts are proceeded consecutively without development process between, there is no alignment key on the chip for the recognition of pattern's position. After the wafer stage moved lot of shots for the exposure of first part and returned to starting position for the exposure of second part, there is still inaccuracy of displacement in the practical operation of exposer. Consequently, in the process of FIG. 1a, when the patterns of first and second parts are exposed, it is very difficult to control their relative overlaying positions. Maybe for this reason, another producing method that can control the alignment of relative overlaying position of two parts is disclosed in the patent, referring to FIG. 1b. 
Comparing FIG. 1b with FIG. 1a, the largest difference is that after the exposure of the first part on reticle is completed, the development process is made to form an alignment key on the chip. After the exposure of first part is completed and the wafer stage returns to the starting position (first shot), the recognition of pattern's position will be made according to the alignment key, and then the exposure of second part is then proceeded. Therefore, the exposing result of FIG. 1b will be more precise than that of FIG. 1a, but comparing with the production time of FIG. 1a, there will be additional time needed for one development and one in/out of exposer in that of FIG. 1b. 
In addition, another optional producing method is also disclosed in the patent, referring to FIG. 1c. The difference from FIG. 1a and FIG. 1b is that after the IDT of first part is produced by exposed, developed, deposited with metal and lift-off, the wafer is then returned to the exposer for the exposure of second part. Since an alignment key has been created on the chip, the pattern recognition will be made first after the wafer is returned to the exposer for exposure of second part, and the following procedure of production is then made. Although more precise overlaying result can be obtained in FIG. 1c, one additional producing procedure of IDT has to be made, which means at least eight more process items in the procedure.
Obviously, the diffraction effects can be reduced by the producing methods disclosed in the prior paten, but there is still great possibility for improvement in the aspects of pattern alignment method and production cycle time.